Refuse clipboard sends when no session is running

root/app/backend/session_manager.py

@@ -620,6 +620,12 @@ def send_keys(self, key_combination: str) -> Dict[str, Any]:
 
     def set_clipboard(self, text: str) -> Dict[str, Any]:
         """Put text on the X11 CLIPBOARD selection; the RDP/VNC client syncs it to the remote side."""
+        with self.lock:
+            running = self.process is not None and self.process.poll() is None
+        if not running:
+            # Nothing would pick it up: reporting success here sent users looking for text
+            # that never reached the remote machine
+            return {"success": False, "message": "No session is running"}
         try:
             # xclip forks to own the selection, so its output must not be piped back to us
             subprocess.run(

root/app/frontend/js/app.js

@@ -234,13 +234,15 @@ function setupEventListeners() {
         const text = document.getElementById("clip-text").value;
         if (!text) return;
         const results = [];
+        let sent = false;
         try {
             const res = await apiFetch("/api/session/clipboard", {
                 method: "POST",
                 headers: { "Content-Type": "application/json" },
                 body: JSON.stringify({ text })
             });
-            results.push(res.ok ? "Sent to Windows clipboard." : `Remote clipboard failed: ${await errorMessage(res, "unknown error")}`);
+            sent = res.ok;
+            results.push(res.ok ? "Sent to the remote session's clipboard." : `Not sent to the remote session: ${await errorMessage(res, "unknown error")}`);
         } catch (e) {
             results.push("Remote clipboard failed: network error.");
         }
@@ -253,6 +255,7 @@ function setupEventListeners() {
             }
         }
         clipStatus.textContent = results.join(" ");
+        clipStatus.classList.toggle("alert-error", !sent);
         clipStatus.classList.remove("hidden");
     });
 

tests/test_session_manager.py

@@ -514,3 +514,27 @@ def test_connect_starts_the_idle_watcher_only_when_enabled(mgr, monkeypatch):
     mgr.connect({"protocol": "vnc", "host": "10.0.0.5"})
     sm.time.sleep(0.05)  # the watcher runs on its own thread
     assert started == [1200]
+
+
+# ----------------- Clipboard helper -----------------
+
+def test_clipboard_send_without_a_session_is_refused(mgr, monkeypatch):
+    calls = []
+    monkeypatch.setattr(sm.subprocess, "run", lambda *a, **k: calls.append(a))
+    result = mgr.set_clipboard("hello")
+    assert result == {"success": False, "message": "No session is running"}
+    assert calls == []  # nothing written to a clipboard nobody reads
+
+
+def test_clipboard_send_after_the_client_exited_is_refused(mgr, monkeypatch):
+    mgr.process = ExitedProc()
+    monkeypatch.setattr(sm.subprocess, "run", lambda *a, **k: None)
+    assert mgr.set_clipboard("hello")["success"] is False
+
+
+def test_clipboard_send_with_a_session_goes_to_xclip(mgr, monkeypatch):
+    calls = []
+    mgr.process = FakeProc()
+    monkeypatch.setattr(sm.subprocess, "run", lambda cmd, **k: calls.append((cmd, k["input"])))
+    assert mgr.set_clipboard("héllo")["success"] is True
+    assert calls == [(["xclip", "-selection", "clipboard", "-i"], "héllo".encode("utf-8"))]